1. Field of the Invention
The present invention relates to methods of making semiconductor devices and in particular to methods of providing ohmic contacts to compound semiconductor layers utilized in semiconductor devices.
2. Description of the Related Art
The fabrication and operation of basic transistor devices is well known. New technologies have developed needs for higher speed and power transistors capable of withstanding extreme operating conditions such as high temperatures, current, and radiation. Silicon carbide devices have the potential to fulfill these needs but have yet to achieve commercial success. One obstacle to using silicon carbide in electronic devices is the difficulty in providing electrical contacts to the device.
Electrical contacts to silicon carbide may be formed by reacting a contact metal with silicon carbide. One such method involves melting an alloy on the surface of silicon carbide. When the alloy melts, it dissolves and reacts with a small portion of the silicon carbide to form a contact. A second method of creating a contact involves laminating the surface of the silicon carbide with a contact metal. When annealed, this metal reacts with the silicon carbide to form an ohmic contact. The first method results in a contact that is too large for use in miniature devices. Annealing temperatures in the second method would be destructive to insulating layers. Both methods are incompatible with semiconductor devices having thin silicon carbide layers because of problems with metal spiking.
In order to prevent metal spiking, barrier layers between the contact metal and silicon carbide may be used. In one method a portion of doped silicon carbide is bombarded with ions to produce a heavily doped barrier region to which contact is made. Alternatively, a silicide barrier layer may be formed during annealing. Both of these methods are impractical for forming contacts to thin silicon carbide layers. Ion bombardment is not feasible for thin silicon carbide layers because the highly doped region is likely to extend through the entire layer and into an underlying layer. Similarly, the formation of a silicide barrier layer may electrically short a thin silicon carbide layer to an underlying layer because the reaction can consume the entire thickness of the thin silicon carbide layer and a portion of the underlying layer.